An exhaust emission control apparatus that purifies an exhaust gas of an internal combustion engine has been known conventionally.
An exhaust emission control apparatus described in a patent document 1 is provided with a supply device (fuel reformation device) that supplies an exhaust passage of an internal combustion engine with a reducing agent and a catalyst (selective reduction type catalyst) that is provided on a downstream side of the supply device in the exhaust passage. The supply device provided by the exhaust emission control apparatus supplies the exhaust passage with gas containing hydrogen, which is generated by reforming fuel, as a reducing agent. When the exhaust gas and the reducing agent, which flow in the exhaust passage, flow into the catalyst, NOx contained in the exhaust gas is reduced to N2.
The supply device provided by the exhaust emission control apparatus described in the patent document 1 supplies the reducing agent to the exhaust passage by the use of the pressure of air introduced into the supply device from an air tank or a compressor. For this reason, when the driving condition of the internal combustion engine is brought into a high load and a gas pressure of the exhaust passage positioned on an upstream side of the catalyst is increased, an amount of the reducing agent supplied to the exhaust passage from the supply device is liable to be decreased by an increase in an energy consumption of the air tank or by an output limit of the air tank or the compressor.
Further, in the exhaust emission control apparatus described in the patent document 1, in the case where the amount of the reducing agent supplied to the exhaust passage from the supply device is increased, there is apprehension that the size of the air tank or the compressor is increased or that a power consumption of the compressor is increased.
In a patent document 2 is disclosed an exhaust emission control system of an internal combustion engine that desorbs nitrogen oxide adsorbed by a catalyst and that purifies an exhaust gas by the use of a reducing agent. In this exhaust emission control system, a desorption amount of the nitrogen oxide desorbed from the catalyst is estimated on the basis of a catalyst temperature, an adsorption amount of the nitrogen oxide, and a flow rate of passing gas passing through the catalyst.
In addition, in this exhaust emission control system, a data table is used for estimating the desorption amount of the nitrogen oxide. The exhaust emission control system can estimate a unique desorption amount of the nitrogen oxide for the catalyst temperature, the adsorption amount of the nitrogen oxide, and the flow rate of the passing gas passing through the catalyst, which are described above, on the basis of the data table.
In general, a catalyst used for the exhaust emission control system includes a catalyst having a plurality of adsorption states. In this catalyst, a desorption characteristic of the nitrogen oxide is different depending on the adsorption state. The inventor focuses on a difference in the desorption characteristic of the nitrogen oxide which varies according to the adsorption state of the catalyst.
The patent document 2 does not focus on the adsorption state of the nitrogen oxide in the catalyst, and the desorption amount estimated by the use of the data table does not reflect the adsorption state of the nitrogen oxide in the catalyst. For this reason, in an exhaust emission control system provided with a catalyst having a plurality of adsorption states, if the desorption amount is estimated by a method disclosed in the patent document 2, there is apprehension that an estimated desorption amount is a value deviated from an actual adsorption amount.
In a patent document 3 is disclosed an exhaust emission control apparatus that uses fuel as a reducing agent and that purifies nitrogen oxide contained in the exhaust gas by a catalyst. When this catalyst is continuously used, the catalyst is degraded. Hence, the exhaust emission control apparatus disclosed in the patent document 3 determines a degree of progress of degradation of the whole catalyst by sensing the temperature of the catalyst and corrects a supply amount of the fuel supplied to the catalyst as the reducing agent.
The inventors have focused on a fact that a catalyst continuously used has both of a portion in which degradation progresses and a portion in which degradation little progresses. As the result, the inventors have found a problem that in the exhaust emission control apparatus of the patent document 3, a decrease in performance of the catalyst caused by the degradation becomes noticeable more than necessary. Describing in detail, in the exhaust emission control apparatus of the patent document 3, the degradation of the catalyst is determined by considering the whole catalyst as one catalyst and the supply amount of the fuel that is the reducing agent is adjusted. For this reason, even if a portion in which degradation little progresses remains in the catalyst, the portion in which degradation little progresses cannot sufficiently perform an operation to purify the nitrogen oxide in the exhaust gas. As the result, the decrease in performance of the catalyst caused by the degradation becomes noticeable.
[Patent document 1] JP 2014-122550 A
[Patent document 2] JP 2002-332835 A
[Patent document 3] JP 2001-59413 A